M.G.
Mine are 3, 2 and newborn. I could not live without the infant seat. I don't know how I would take the newborn around without it, waking him up, dealing with the cold weather. Infant seats can be attached to those cloth things that go over baby to keep him/her warm. At least in Chicago that's a plus! Also, the seat clips into my stroller, since the seats are TOO heavy to carry very far.
You'll need a convertible seat anyway, right? And it lasts until 40 pounds or more. Buy one when the time comes, as they change all the time and get better. The best I found is Evenflo Triumph Deluxe Advanced, something like that. It has the adjuster on the side, which is a breeze to use. I found my babies can make it to 8-10 months before I use the convertible seat.